ECE1750, Spring 2017 Week 12 – Motor Drives and Other ...akwasins/power electronics...

ECE1750, Spring 2017

Week 12 – Motor Drives and Other ApplicationsApplications

1

Three-Phase Induction Motors

• Reliable

Rugged• Rugged

• Long lived

• Low maintenance

• Efficient

(Source: EPRI Adjustable Speed Drives Application Guide)

The rotor also has “windings”

St t i di l ki f

• Current convention: Positive

Stator winding looking from the front (3-phase system)

Current convention: Positive current….

• Goes “in” in terminal A

G “ t” f t i l• Goes “out” from terminal a


• A 3-phase system of circulating currents in the stator creates a rotating (varying) magnetic field.• The varying rotating magnetic field induces a 3-phase system of currents in the rotor.• The 3-phase system of rotor currents creates a rotating magneticThe 3 phase system of rotor currents creates a rotating magnetic field.• The interaction of both rotating magnetic fields makes the rotor to turn (it is like one magnet pushing into another).

Per-Phase Representation(assuming sinusoidal steady state)( g y )

Ohmic losses = mechanical power output

The shunt inductance term represents the flux in the machine pair gap between rotor and stator.

The flux is proportional to Vs / f

(This characteristic will lead to an important conclusion in a few slides…. Stay tuned)

Source: Ned Mohan’s power electronics book

Torque-Speed Characteristics

s rli s r

s

slip s

speed of the stator rotatingmagnetic field

s magnetic field

speed of the rotorr

.slf s f

• The linear part of the characteristic is utilized in padjustable speed drives


Acceleration Torque at Startup

racc em load

dJ T T T

dt

• Intersection represents the equilibrium pointp q p


0acc em loadT T T

Slip frequency (about 5% of no load speed), so induction motors are almost constant speed devices

B

At no load, the motor spins at grid frequency, divided by the number of pole pairs (in each phase).

Usually this is 3600 / 2 = 1800RPM

High slip corresponds toHigh slip corresponds to low efficiency

Speed Adjustment by Changing the Input Voltage


Speed Adjustment by Changing the Line Frequency

It’s much more effective to reduce operating speed by lowering the frequency of the supply voltage. q y pp y g

But how?

Per-Phase Representation(assuming sinusoidal steady state)( g y )

Ohmic losses = mechanical power output

The shunt inductance term represents the flux in the machine pair gap between rotor and stator.

The flux is proportional to Vs / f

Because of the shunt inductance term, we must reduce the ,applied voltage magnitude in proportion to applied frequency to avoid serious saturation of the iron near the air gap

This is what is called “Constant Volts per Hertz Operation,” which is p pthe standard operating mode for ASDs


PWM-VSI System

A three phase DBR

A three-phase inverter

A three-phase DBR

• Diode rectifier for unidirectional power flowSource: Ned Mohan’s power electronics book

Three-Phase Inverter(called a six-pack)

• Three inverter legs; capacitor mid-point is fictitious g p p


Three-Phase PWM

WaveformsNOTE: ModulationNOTE: Modulation index is different from that on single-phase inverters In three-inverters. In three-phase inverters:

V ,3 2

2

ph rmsph

dc

Vm V


Three-Phase Inverter Harmonics

Compare with slide 22 in “Inverter Basics presentation”


Three-Phase Inverter Output

Li d d l ti• Linear and over-modulation rangesSource: Ned Mohan’s power electronics book

Adjustable-Speed Motor Drives (ASDs)


Some Prices for Small 3-Phase, 460V Induction Motors and ASDs

Power Motor ASD

10kW $750 $2,0000 $ 50 $ ,000

100kW $5,000 $15,000

$50 - $75 per kW $150 - $200 per kW

For Comparison, Conventional Generation: $500 - $1,000 per kW

Solar: $5,000 - $8,000 per kW (but the fuel is free forever!)So a $5,000 $8,000 pe (but t e ue s ee o e e )

Pump Application: Adjustable Flow ratej

Bad news – inefficient!

Equivalent to reducing the output voltage of a DBR with a series resistor

Payback in energy savings is about 1 year

• Fixed versus adjustable speed drivej p


Torque Speed Characteristics at various Frequencies of Applied Voltage

For a constant torque load

• The air gap flux is kept constantg p p


Adjusting Speed of a Centrifugal Loadj g g

• The load torque is proportional to speed squared


Operating regionsg g

Pull-out torque line

V/f = constant


Frequency at StartupAt startup f=fsl so this fictitious resistance is a short circuit and the (Rotor portion of

current is high( pthe equivalent

circuit)

So the current can be reduced by reducing the input frequency f,

which, in turn, equals fsl at start

An important property of ASDs is the ability to “soft start” a

, , q slup (because the ratio with respect

to fsl tends to increase)

• The torque is limited to limit current draw (e.g. max startup current 150% of the rated current)

motor by reducing the applied frequency to a few Hz

max startup current 150% of the rated current)

Source: Ned Mohan’s power electronics book ,

startstart sl rated

rated

Tf f

T

Braking(Regenerative braking)

Initial operation point

(Regenerative braking)

(Motor)

Fi l ti i t E bFinal operation point(Generator)

Energy can be dumped in

resistances or used to recharge batteries g(e.g. hybrid cars)


Improving Energy Efficiency of Heat Pumpsg gy yHow does inserting an ASD save energy in single-phase applications?

9 %

Some losses

But a three-phase motor is 95% efficient, compared to 80% efficiency for a single-phase motor

• Used in one out of three new homes in the U.S.


Loss Associated with ON/OFF Cyclingy gThe big efficiency gain is here

• with conventional air conditioners, the first few minutes after start-up are very inefficient as the mechanical system reaches steady-state

• with ASDs, the air conditioner speed is lowered with demand, so that there are fewer start-ups each day

• The system efficiency is improved by ~30 percenty y p y p


Electronic Ballast for Fluorescent Lamps

• Fluorescent lamps operated at ~40 kHz save energyp p gy


Induction Cookingg

• Pan is heated directly by circulating currents – increases efficiency (and can make the panefficiency (and can make the pan levitate)


http://web.mit.edu/cjoye/www/photo/0610MrMagnet/

Industrial Induction Heating

• Used in steel industry


• Used in steel industry

HVDC Transmission

• There are many such systems all over the world

•Issue in power electronics: switching frequency goes


p g q y gdown as voltage goes up (and filters become bulkier)

HVDC Poles

• Each pole consists of 12-pulse convertersp p


HVDC Transmission: 12-Pulse Waveforms


Reducing the Input Current DistortionReducing the Input Current Distortion

Lik DBR t (hi h di t ti )Like DBR current (high distortion)


Power-Factor-Correction (PFC) Circuit( )

The boost converter is operated toThe boost converter is operated to make the DBR current look sinusoidal on the AC side

To be sold in Europe, this is a necessary feature in high-current single-phase power electronic loads

It also permits more power to be drawn from conventional wall outlets because the harmonic currents are minimal


Power-Factor-Correction (PFC) Circuit( )

The boost converter is instructed to

“ l ” h th t i b l th“close” when the current is below the sinewave envelope, and

“open” with the current is above the sinewave envelope

• Operation during each half-cycle

p

p g y


Typical telecom rectifier schematic

Isolationbarrier

High-frequencytransformer

Power Electronics Has Made Wind Farms Possible

The choices used to be

• Use an efficient induction generator, which has very poor power factor, or

• Use a synchronous generator but constantly fight to• Use a synchronous generator, but constantly fight to synchronize the turbine speed with the grid.

Now,

• Either use a DC bus and inverter to decouple the generator and grid AC busses, or

• Use a doubly fed induction motor operate the wind turbine at• Use a doubly-fed induction motor, operate the wind turbine at the max power speed, and use power electronics to “trick” the wind generator into producing grid-frequency output. This is what you see in West Texaswhat you see in West Texas.

ECE1750, Spring 2017 Week 12 – Motor Drives and Other ...akwasins/power electronics...

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